1. The Field of the Invention
The present invention relates to electrical connections in semiconductor chip technology. More particularly, the present invention relates to formation of substantially dielectric-free bonding pads for semiconductor chips. In particular, the present invention relates to a method of removal of oxide from a metallic bonding pad through the use of an oxidizing compound that catalyzes the reductive polymerization of an electrically-conductive monomer.
2. The Relevant Technology
In the microelectronics industry, a substrate refers to one or more semiconductor layers or structures which include active or operable portions of semiconductor devices. In the context of this document, the term xe2x80x9csemiconductive substratexe2x80x9d is defined to mean any construction comprising semiconductive material, including but not limited to bulk semiconductive material such as a semiconductive wafer, either alone or in assemblies comprising other materials thereon, and semiconductive material layers, either alone or in-assemblies comprising other materials. The term xe2x80x9csubstratexe2x80x9d refers to any supporting structure including but not limited to the semiconductive substrates described above. The term xe2x80x9csemiconductor substratexe2x80x9d is contemplated to include such structures as silicon-on-insulator and silicon-on-sapphire.
Electrical interconnections, such as for flip chip or printed circuit board interconnections that are to connect a semiconductor die pad circuit to a supporting substrate, have historically been made with plating methods and reflow solder attachment methods. Electrically conductive epoxy has also been used to make the electrical interconnections. The use of electrically conductive epoxy interconnections typically have problems with high contact resistance. For example, when used to contact an aluminum bonding pad, which is a semiconductor industry standard, a nonconductive oxide-coated surface forms on the aluminum bonding pad under ambient conditions. Because of the nonconductive oxide, electrically conductive epoxies on otherwise bare aluminum form an interconnect having an unacceptably high electrical resistance at the contact. The contact electrical resistance may be in a range from about 100 ohms to several millions of ohms.
Efforts to reduce contact electrical resistance include using a precious metal such as gold. Gold, however, due to its cost makes mass production of gold bonding pads uneconomical. Additionally, the plating of gold as a semiconductor chip or printed circuit board bonding pad is a difficult process that in and of itself is expensive and time consuming. For example, electroless plating processes are difficult to achieve. Hence, the semiconductor substrate or printed circuit board must be electrically connected to a power supply in order to achieve gold plating.
Other methods to reduce the contact electrical resistance caused by the oxide formed upon the bonding pad include chemical and/or mechanical removal, typically by abrasive means. Although removal of oxides is achievable, the contact will immediately re-oxidize to form a native oxide film unless the contact is in a protected environment.
Another method that has been employed to resist oxidation of metal bonding pads, for example bonding pads made of aluminum, includes the deposition of a strong oxidizer directly upon the bonding pad. When a strong oxidizer is placed directly upon the bonding pad, it forms an oxide husk upon the bonding pad. The oxide husk must be subsequently consumed completely because it otherwise acts as an electrical insulator. Where a stoichiometric excess is needed to get a reaction to provide a sufficient product, formation of excess oxide husk causes additional challenges for its ultimate removal. Removal of any native oxide from the bonding pad will be undermined by the residual presence of the oxide husk formed from the strong oxidizer due to its function as an electrical resistor.
In one prior art attempt to solve the problem of removing the native oxide film, a strong oxidizer is contacted to the bonding pad and an oxide husk thereof deposits upon the native oxide film. FIG. 1 illustrates a cross-sectional view of a portion of a bonding pad structure 10 that includes an aluminum bonding pad 12 with a native oxide film 14 upon a free surface 16. A manganese oxide husk 18 is next formed upon native oxide film 14 by contact with a strong oxidizer such as potassium permanganate. Next, an electrically conductive polymer 20 is formed upon the manganese oxide husk. This method presents several problems. First, the proper amount of the strong oxidizer making contact with the native oxide film 14 is critical. Formation of an excessive amount of manganese oxide husk 18 upon native oxide film 14 will make its removal difficult during formation of electrically conductive polymer 20. Thus, a dielectric interlayer between aluminum bonding pad 12 and any external wiring such as a conductive bump for flip chips or a soldering wire will substantially hinder electrical conductivity. Second, due to the nature of the strong oxidizer, additional oxidation of aluminum bonding pad 12 may occur to thicken native oxide film 14. Third, if an insufficient amount of electrically conductive polymer 20 is formed above manganese oxide husk 18, an insufficient quantity of the metal oxide will be reduced, including any of the native oxide film of the bonding pad. Finally, forming too much of electrically conductive polymer 20 upon manganese oxide husk 18 will prevent complete formation of the polymer such that electrical conductivity through an uncompleted polymer layer will be substantially reduced.
What is needed in the art is a method of forming a substantially oxide-free bonding pad without the problems of the prior art. What also is needed is a method of forming a substantially oxide-free bonding pad that contains no residual substance that was used to remove native oxide and that contains no such substance that might simultaneously interfere with the electrical conductivity of the bonding pad.
The present invention is drawn to a method of lowering the net resistivity of an interconnect by depositing a monomer layer upon a metal bonding pad, the treatment thereof to cross-link the monomer to form an electrically conductive polymer, and simultaneously, the substantial reduction of metal oxide to zero valent metal.
In the method of the present invention, deposition of a monomer layer in a solvent, volatilization of the solvent, and contact with a strong oxidizer such as a potassium permanganate allows for the use of the strong oxidizer without the hindrance of having to deal with a manganese oxide husk on the surface of the aluminum bonding pad.
The present invention includes cleaning at least one bonding pad of a chip package or printed circuit board by any appropriate chemical rinse so as to remove harmful particulates and other pollutants whether organic or inorganic. Following cleaning of the bonding pad, a monomer is selected and contacted with the bonding pad. The monomer may be solubilized with any preferred type of solvent such as water or an organic solvent. Care should be taken not to prematurely cause cross-linking of the monomer during the process of volatilizing at least some of the solvent.
Preferably, the chemical qualities of the monomer will include the tendency to be a reducing agent to the native oxide film of the bonding pad. By selecting a monomer that tends to reduce rather than to oxidize, the problem of thickening the native oxide film is avoided. Another preferred chemical quality of the monomer and its polymer after cross-linking, is that it will act as a protective coating to the chip package for substantially all further processing. In particular, the monomer or its cross-linked polymer will act as a protective coating to the chip package during an acid dip in an oxidizer solution such as acidic KMnO4.
Following the formation of the monomer into a monomer layer, a strong oxidizer film is deposited upon the monomer layer. With a film of a strong oxidizer in place upon the monomer layer, conditions are selected to cause the strong oxidizer to cross-link the monomer layer. In the present invention, the method of cross-linking the monomer layer may be done for example by dipping the assembly of bonding pad, native oxide film, monomer layer, and strong oxidizer film in an acid solution.
Alternatively, the strong oxidizer may first contact the monomer layer as oxidizer dissolved in an acidic solution by dipping the bonding pad or by spraying the acidic oxidizer solution. Under such conditions, the strong oxidizer is dissolved in an aqueous acidic solution and the bonding pad is immersed into the solution or the solution is sprayed thereupon. Under most conditions, stirring of the solution is not carried out and dipping is preferred. By not stirring, only a film of strong oxidizer immediately adjacent to the monomer layer is substantially active upon the monomer layer.
Upon contact with an acid environment, the strong oxidizer begins to cross link the monomer layer making the cross linking monomer layer more electrically conductive. Simultaneously, with the cross linking of the monomer layer, because of its tendency to be a reducing agent, or at least significantly less prone to oxidize the bonding pad than the strong oxidizer, the native oxide film is reduced to zero valent metal and the bonding pad is simultaneously covered by the cross-linked polymer.
Below an electrode potential difference of about 1 V between the strong oxidizer and the monomer layer, oxidation of the monomer layer and the subsequent reduction of the native oxide film upon the bonding pad may be assisted by controlling concentration of the strong oxidizer and/or the monomer cross linking reaction temperature. Thus, where a lower electrode potential difference is required, a higher oxidizer concentration, a more acidic solution, an increase in the reaction temperature, or a subset thereof will assist in appropriate and substantially complete cross linking of the monomer layer. The oxidation of the monomer layer through cross linking will both cause its polymer to be more electrically conductive, and will elimnate the native oxide film from the metallic surface of the bonding pad. Thereby, the net electrical resistivity of the interconnect will be lowered.
Following cross linking of the monomer layer and reduction of the native oxide film according to method of the present invention, the surface of the chip package is cleaned according to know methods in the art.
Distinct advantages are exhibited by the method of the present invention. In the method of the present invention, a stoichiometric excess of strong oxidizer provides no hindrance to achieving a final article of bonding pad that is substantially free of oxides.
Where conditions of cross linking the monomer layer are carried out by dipping the bonding pad into a solution of acidic oxidizer, the degree of stoichiometric excess is of substantially no concern. The technique of providing a stoichiometric excess of the oxidizer in the prior art caused the problem of having excess of metal oxide husk remain directly upon the bonding pad that resulted in increased resistivity. In the present invention, where a dip is carried out, a process engineer may use substantially any stoichiometric excess of strong oxidizer that is desired in order to properly cross link the monomer layer. Thereby, where the bonding pad is aluminum or an alloy thereof, it is assured that the monomer layer during cross linking will substantially reduce all alumina to metallic aluminum
These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.